1. Field of the Invention
The present invention relates to an improved temperature characteristic compensating circuit that uses analog processing to compensate for a temperature characteristic of a signal processing circuit of a photosensor used in a camera or a camera flash or the like, and an improved semiconductor integrated circuit that contains the temperature characteristic compensating circuit.
2. Description of Related Art
In an analog circuit, when logarithmically compressing the output of a photosensor using a diode and carrying out signal processing on the resulting output, due to the temperature dependence of the I-V (current-voltage) characteristic of the diode, the output voltage is proportional to the absolute temperature. The temperature characteristic of the output of the photosensor is thus compensated for using an external thermistor and an external resistor as shown in FIG. 3, and then the signal processing is carried out after that.
In FIG. 3, reference numeral 21 designates the photosensor, and 22 designates a diode that carries out logarithmic compression of the output current from the photosensor 21 and converts the current into a logarithmically compressed voltage in cooperation with an operational amplifier 23. Reference numeral 24 designates a diode, 25 an operational amplifier, and 26 a constant current source. The diode 24, the operational amplifier 25 and the constant current source 26 are for compensating for the dark current of the diode 22.
When the dark currents of the diodes 22 and 24 are equal, the output voltage after the dark current compensation is (kT/q)ln(Ip/Iref), wherein k represents Boltzmann""s constant, T the absolute temperature, q a unit charge, Ip the photocurrent, and Iref the above-mentioned constant current.
Because the output is proportional to the absolute temperature T, before carrying out the signal processing, a gain that is inversely proportional to the absolute temperature T is applied using an external thermistor 27 and an external resistor 28, thus producing an output that does not vary with temperature.
Because the temperature compensation is carried out using the external thermistor 27 and the external resistor 28, external terminals 30 and 31 that are connected to an operational amplifier 29 of an IC (semiconductor integrated circuit) containing the photosensor 21 etc. are required, as shown in FIG. 3.
The IC is generally composed of transistors (including field effect transistors and diodes), resistors and capacitors. Incorporating a thermistor having a negative temperature characteristic into the IC is problematic, and hence an external thermistor has to be used.
If temperature characteristic compensation could be carried out without using an external thermistor, then the component mounting area could be reduced accordingly and external terminals would become unnecessary, resulting in a smaller IC.
It is an object of the present invention to provide a temperature characteristic compensating circuit that is capable of carrying out temperature compensation of a signal that varies in proportion to absolute temperature by analog processing and without using a thermistor, to thereby enable use of a smaller IC, and a semiconductor integrated circuit that contains the temperature characteristic compensating circuit.
In one aspect of the present invention, the temperature characteristic compensating circuit comprises a first current source that supplies a first current that is proportional to the absolute temperature and inversely proportional to the resistance value of a first resistor, a second current source that supplies a second current that is inversely proportional to the resistance value of a second resistor, a first circuit that carries out logarithmic compression of an input voltage using the first current as a bias current, and a second circuit that carries out logarithmic expansion of the logarithmically compressed voltage using the second current as a bias current. The gain of the logarithmically expanded voltage relative to the input voltage is proportional to the ratio of the second current to the first current. As a result of the above, a temperature characteristic compensating circuit that does not use an external thermistor but nevertheless gives a gain inversely proportional to absolute temperature can be formed.
In the above constitution, the ratio of the resistance value of the first resistor to the resistance value of the second resistor is constant regardless of temperature changes.
In a typical preferred form, the first circuit and the second circuit each comprise transistors, diodes and resistors.
In another aspect of the present invention, the temperature characteristic compensating circuit comprises a first current source that supplies a first current that is proportional to absolute temperature and inversely proportional to a resistance value of a first resistor, a second current source that supplies a second current that is inversely proportional to a resistance value of a second resistor, a voltage-current converting circuit that converts an input voltage into a current, using a third resistor, and using the first current as a bias current, a logarithmic compression circuit that passes an output current from the voltage-current converting circuit through a diode, thus obtaining a logarithmically compressed voltage, a logarithmic expansion circuit that comprises a differential transistor using the second current as a bias current, and a current-voltage converting circuit that passes, through a fourth resistor, an output current obtained from the logarithmic expansion circuit by inputting an output from the logarithmic compression circuit into the logarithmic expansion circuit, thus obtaining an output voltage.
Preferably, the first, second, third and fourth resistors each have the same temperature characteristic.
Further, according to the present invention, there is provided a semiconductor integrated circuit having the temperature characteristic compensating circuit according to either of the aspects of the present invention.